This invention relates to the fabrication of polarization retaining single-mode (PRSM) optical fibers and more particularly to the fabrication of preforms from which fibers having elliptically-shaped cores can be drawn.
In many applications of single-mode optical fibers, e.g. gyroscopes, sensors and the like, it is important that the propagating optical signal retain the polarization characteristics of the input light in the presence of external depolarizing perturbations. This requires the waveguide to have an azimuthal asymmetry of the refractive index profile.
One of the first techniques employed for improving the polarization performance of single-mode fibers was to distort the symmetry of the core. One such optical fiber is disclosed in the publication by V. Ramaswamy et al., "Influence of Noncircular Core on the Polarization Performance of Single Mode Fibers", Electronics letters, Vol. 14, No. 5, pp. 143-144, 1978. That publication reports that measurements made on such fibers indicated that the noncircular geometry and the associated stress-induced birefringence alone were not sufficient to maintain polarization in single-mode fibers.
Fiber cores having a relatively high aspect ratio are required to obtain adequate polarization retaining properties. A high core/clad .DELTA. also improves these properties. Techniques which have been developed for improving core ellipticity are subject to various disadvantages. Some techniques are not commercially acceptable because of their complexity. Double crucible techniques result in fibers having relatively high attenuation. Some techniques employ very soft glasses for certain fiber portions, and those soft glasses are detrimental to the propagation of light at long wavelengths where the core glass would normally experience extremely low attenuation. Soft glasses can also complicate the fusion splicing of fibers, since the soft glass flows too readily when the fibers are heated during the splicing operation.